Unifi-python-api

- added `list_fingerprint_devices()` method to list fingerprints for clients devices, contributed by dream-rhythm, 213
- minor code clean up

- further code clean up and refactoring to pass more phpstan tests

- fixes issue with `curl_init()` returning CurlHandle class objects instead of curl handlers with PHP 8 and higher

- removed unnecessary type casting in several methods
- fixed return type hinting in several methods where return type needed to be array|bool instead of just array
- relaxed parameter type hinting in the constructor to allow for null values for several parameters that have sensible defaults, reported by pbksol
- using phpstan, the class passes level 5 checks

- PHP 7.4 is now the minimum required version
- updated the code for 7.4. specific features such as parameter type hinting, return types, etc.
- added the ability to the constructor to change the key that is used to store the unificookie in the session,
(`$_SESSION['unificookie']` is used by default), this is useful when running multiple applications on the same server

- added a `accept: application/json` header to the login requests
- re-added support for cookies when working with a UniFi OS-based controller